1. Field of the Invention
Embodiments of the present invention relate, in general, to systems and methods for enhanced 1-Hop Dynamic Frequency Hopping Communities, and particularly to algorithms to coordinate channel hopping and increase Dynamic Frequency Hopping Communities' channel efficiency.
2. Relevant Background
Dynamic Frequency Hopping (“DFH”) is a technique that incorporates non-traditional dynamic channel allocation with slow frequency hopping. The main objective in DFH is to provide capacity improvements through the addition of interference avoidance that are higher than those provided by conventional frequency hopping while preserving interference averaging characteristics of conventional frequency hopping in order to provide robustness to rapid changes in interference.
Generally, the key concept behind this intelligent type of frequency hopping is to adjust or create frequency hopping patterns based on interference measurements. DFH uses slow frequency hopping and adaptively modifies the utilized frequency hopping pattern based on rapid frequency quality measurements, sometimes referred to as Quality of Service (“QoS”) measurements. This technique combines traditional frequency hopping with dynamic channel allocation, where a channel is one frequency in a frequency hop pattern. The continuous modification of frequency hop patterns is based on measurements representing an application of dynamic channel allocation to slow frequency hopping. Modifications are based on rapid interference measurements and calculations of the quality of frequencies used in a system by all Customer Premise Equipments (“CPEs”) and Base Stations (“BSs”). The target of these modifications is tracking the dynamic behavior of the channel quality as well as of interference. One significant application of DFH is found in the operation of what is referred to in the art as a Wireless Regional Area Network (“WRAN”).
A WRAN cell consists of a BS and the associated CPEs that communicate to the BS via a fixed point-to-multi-point radio air interface (i.e. an antenna). WRAN operations need to satisfy two apparently conflicting requirements: assure the QoS satisfaction for WRAN services while providing reliable and timely frequency spectrum sensing. Current sensing requirements state that incumbent signals shall be detected by WRAN devices with no more than a 2 second delay. Thus a WRAN cell must perform sensing on a working channel every 2 seconds. A channel that is to be sensed cannot be used for data transmission, thus a cell operating consistently on a single channel must interrupt data every 2 seconds for sensing. Such a non-hopping mode leads to periodic interruptions and can significantly decrease system throughput and impair QoS. The solution to this problem, as will be appreciated by one skilled in the art, was DFH.
As previously described, DFH differs from conventional frequency hopping in the way the patterns are built. Instead of using random or pre-defined repetitive hopping patterns, DFH patterns are generated for active users on the fly. In this manner the hopping patterns can be adjusted to adapt to interference changes. The basic idea behind creating the patterns is to choose the best frequency for each hop. This best frequency corresponds to the frequency that is interfered with the least. DFH thus requires continuous estimation and measurement of the interference at every frequency from every single hop of a pattern. At each hopping instant, the BS or the CPE measures the QoS of each frequency, filters the measurement to average out the instantaneous Rayleigh fading effects, and then sends the data using the ‘best’ frequency chosen according to some quality selection criteria. Typically the hopping patterns for users within the same cell are orthogonal. The performance of an established link is monitored, and upon the performance dropping below a threshold, a better hopping pattern is generated.
In DFH communication, components of a WRAN cell (the CPEs) hop over a set of channels. During operation on a working channel, sensing is performed in parallel on the intended next working channels. After 2 seconds, a channel switch takes place: one of the intended next working channels becomes the new working channel, and the channel previously used is vacated. Hence, an interruption is no longer required for sensing. Obviously, efficient frequency usage and mutual interference-free spectrum sensing can only be achieved if multiple neighboring overlapping WRAN cells operating in the DFH mode coordinate their hopping behavior.
As described a WRAN cell operating in the DFH mode uses the working (in-band) channel for data transmission and performs spectrum sensing on out-of-band channels simultaneously. This operation is referred to as Simultaneous Sensing and Data Transmissions (SSDT). Guard bands between the in-band and out-of-band channels are allocated to mitigate adjacent interference caused by data transmission to the out-of-band sensing. An out-of-band channel sensed to be vacant is considered to be validated. A WRAN system in the DFH mode thus dynamically selects one of the channels validated in a previous operation period for data transmission in the next operation period.
A DFH Community is a non-empty set of neighboring WRAN cells following a common protocol that supports a coordinated DFH operation in order to ensure mutual interference-free channel sensing and to minimize the channel usage, applying the DFH phase-shifting. A DFH Community has one leader and, possibly, some community members. One definition of a DFH Community, as is known to those skilled in the art, is that one-hop BS neighbors create a DFH Community meaning that each WRAN cell in a community is within one-hop of the other members of the cell. A priority value is used to elect the DFH Community leader from those members and the elected leader decides when and which channel to hop among the available channel set for each community member. The community members hop among the same available channels according to the leader's decision in a synchronized fashion. Thus the DFH Community leader is responsible for decisions about community membership, calculating the hopping patterns (phase-shifting sequences) for all members and distributing this information within the community. Members provide the leader with their neighborhood and channel availability information, i.e. information about their sensing results and observed channel usage of the neighboring WRAN cells. Within the DFH Community, hopping information does not change as long as the community is stable (i.e. no new member arrives or existing member departs). Using this method hopping collisions can be avoided and real-time inter-BS communication is not necessary.
DFH Community is thus a concept introducing coordination among cells. The key idea of a DFH Community is that neighboring WRAN cells form cooperating communities which choose their hopping channels and perform DFH operation in a coordinated manner. As discussed above, DFH Community leads to a better QoS and throughput behavior while requiring a modest amount of channels for hopping. A DFH Community enables coexistence of multiple WRAN cells and can also be used to coordinate channel usage of cells operating in the non-hopping mode. Moreover, coordinated channel hopping can give WRAN cells more time to do channel sensing and increase channel efficiency.
It has been shown that only N+1 vacant channels (i.e., channels free of both incumbents and other WRANs) are needed under certain conditions to ensure cells can operate without collisions.
FIG. 1 illustrates the Phase-shifting DFH operation of three, N=3, overlapping WRAN cells over four, (N+1)=4, vacant channels as is known in the prior art. Each WRAN cell shifts its DFH operation phase 110 by one Quiet Time (“QT”) 120 against the operation phase of the previous WRAN cell as shown in FIG. 1. For instance, WRAN2 130 on channel D 140 shifts its operation by one QT 120 against the operation of WRAN1 150 on channel A 160, and WRAN3 170 on channel C shifts by one QT 120 against that of WRAN2 130 on channel D 140. During a QT 120, channel sensing is performed. This implies that a QT 120 has to be at least equal to the minimum time required for reliable channel sensing.
Thus a set of N overlapping cells can operate continuously using (N+1) channels (Channels A, B, C, and D in the example presented in FIG. 1) as long as the length of a single transmission is larger than the product N*QT. Imposing the above explained hopping pattern of time shifted jumps is, however, only possible in case of strict coordination within the DFH Community.
Defining a DFH Community as a number of WRAN cells being one-hop neighbors limits channel efficiency. Redefining the community as WRAN cells that are one-hop neighbors of the leader can enhance efficiency, but such a definition presents several challenges including deciding whether to accept a one-hop neighbor WRAN cell as a community member and how to calculate the hopping mode for each community member. These and other issues are addressed by the present invention.